Organic solar cells (OSC) or organic photovoltaics (OPV) based on π-conjugated polymers (e.g. poly-3-hexylthiophene (P3HT)) and fullerene derivatives (e.g. [6,6]-phenyl C61 butyric acid methyl ester (PCBM)) have attracted attention over the past decades because they may provide a cost-effective route to wide use of solar energy for electric power generation.
These organic semiconductors have the advantage of being chemically flexible for material modifications, as well as mechanically flexible for the prospective of low-cost, large scale processing such as screen-printing or spraying on flexible substrates. The world's next generation of microelectronics may be dominated by “plastic electronics” and organic solar cells are expected to play an important role in these future technologies.
The photovoltaic process in organic solar cell devices consists of four successive possesses: light absorption, exciton dissociation, charge transport, and charge collection. Absorption of a photon creates an exciton (bounded electron-hole pair). The exciton diffuses to the interface of two different components, where exciton dissociation, or charge separation, occurs, followed by positive charges (holes) moving to the anodes and negative charges (electrons) to the cathode.
Several parameters determine the performance of a solar cell, namely, the open-circuit voltage (Voc), short-circuit current (Isc), and the so-called fill factor (FF). The overall power conversion efficiency η is defined as η=(FF)*(Isc Voc)/Pm. Over the past decade, OPV efficiency has been significantly improved to over five percent in single cell and one percent in submodules owing to a better understanding of device physics, optimization of device engineering, and developments of new materials.
However, most of such organic solar cell devices are developed in laboratories with fabrication processes involving spin-coating for the photoactive layer and the use of high vacuum to deposit the metal cathode. This conventional technique limits the real potential of organic solar cells in the commercial market due to the high cost of manufacturing using high vacuum.
Recently, world-wide research efforts have been made to develop transparent contacts based on modified Poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) solution. Y. Liang et al., Development of New Semiconducting Polymers for High Performance Solar Cells, J. Am. Chem. Soc., V. 131, 56-57 (2009). For large scale production, screen printing (S. Shaheen et al., Fabrication of Bulk Heterojunction Plastic Solar Cells by Screen Printing, Appl. Phys. Lett., V. 79, 2996-2998 (2001)) and ink jet printing (T. Aernouts et al., Polymer Based Organic Solar Cells Using Ink-Jet Printed Active Layers, App. Phys. Lett., Vol 92, 033306 (2008)) have been demonstrated mostly in OPV single cells.
Spraying methods, such as that described in Lim et al., have also been attempted. Lim et al., Spray-Deposited Poly(3,4-ethylenedioxythiophene:Poly(styrenesulfonate) Top Electrode for Organic Solar Cells, App. Phys. Lett., V. 93, 193301 (2008). However, such methods spray a thick layer of PEDOT:PSS to replace the need for metal cathode deposition using high vacuum. This thick layer of PEDOT:PSS sacrifices transparency, which is needed in certain application such as window technology. In fact, the thickness of the PEDOT:PSS layer produced by the method described in Lim et al. is over 2 μm. When thickness is over 1.26 μm, the transparency is below 1% (completely opaque), making Lim's method ineffective for producing transparent or even semi-transparent contacts for organic solar cells.